1. Field of the Invention
The present invention relates to a liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices have been used as the displays of cellular phones, smart phones, personal digital assistants (PDAs), personal computers and so on utilizing their features of being flat, lightweight, and low in power dissipation. The addition of an external input function, such as a touch sensitive screen or pen input, to the liquid crystal display devices has extended their uses besides display. To add the external input function, however, it is required to mount new components on the liquid crystal display devices. Problems therefore arise in that the thickness and weight of the display device increase and the cost increases because of the addition of new components and new assembly steps.
Heretofore, a drive circuit has been installed on the array substrate by being externally attached to it. In recent years, however, a technique has been developed which manufactures a drive circuit into the array substrate by forming thin-film transistors (TFTs) for the drive circuit on the array substrate in forming pixel switching TFTs on the array substrate. This allows the manufacturing cost, the weight and the thickness of the liquid crystal display device to be reduced.
By manufacturing a photosensor into each pixel of the array substrate utilizing the aforementioned technique, the liquid crystal display device is allowed to obtain an image read function and an input function. This technique is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 2007-304519 by way of example. For example, backlight from a source placed outside the countersubstrate passes through a pair of substrates and is then directed onto a target of image reading placed outside the array substrate and reflected by that target. The photosensors can detect the intensity of the reflected light from the target. The liquid crystal display device can also obtain the input function by detecting the intensity of external light with the photosensors.
As a display mode of the liquid crystal display device, the vertical alignment (VA) mode has been in wide use. The liquid crystal layer is formed from a liquid crystal material which is negative in dielectric anisotropy. The liquid crystal display device can provide sufficient black display and increase contrast by aligning liquid crystal molecules perpendicularly to the substrate using the VA mode and setting the double refractive index of the liquid crystal layer to nearly zero.
Above all, a liquid crystal display device of multi-domain vertical alignment (MVA) mode in which the direction of inclination of liquid crystal molecules inside each pixel is divided into two or more regions with a structure is excellent in display quality (contrast and so on) and has a wide viewing-angle characteristic.
However, when a photosensor is placed inside each pixel, that area in one pixel which can be used for image display will become small. A display image in this case will become darker than one when no photosensors are placed.
With the MVA mode in which a wide viewing angle and a high contrast can be expected, one or both of the array substrate and the countersubstrate need a liquid crystal alignment control structure which is formed of dielectric projections or indium tin oxide (ITO) slits.
In the neighborhood of that structure, liquid crystal molecules little react and contribute to transmissive display, causing a problem that the display image becomes dark. When the dielectric projections are used as the liquid crystal alignment control structure, light leaks to inclined portions of the projections due to the influence of aligned liquid crystal molecules, causing a problem of low contrast ratio.